J Patient Saf ● Volume 8, 2012
Hess et al ● Trends in CT Utilization Rates
Original Article
TRENDS IN COMPUTED TOMAGRAPHY UTILIZATION RATES: A LONGITUDINAL
PRACTICE-BASED STUDY
Erik P Hess MD, MSc1,2, Lindsey R Haas MPH3, Nilay D Shah PhD2,3, Robert J Stroebel MD4, , Charles R. Denham
MD5, Stephen J. Swensen MD, MMM6
Abstract
INTRODUCTION
Objectives: Computed tomography (CT) use has increased dramatically
over the past two decades, leading to increased radiation exposure at the
population level. We assessed trends in CT use in a primary care (PC)
population from 2000 to 2010.
Methods: Trends in CT use from 2000 to 2010 were assessed in an
integrated, multi-specialty group practice. Administrative data were
used to identify the impaneled primary care population and all CT
imaging procedures. Utilization rates per 1,000-paneled patients and CT
rates by type and medical specialty were calculated.
Results: Of 179,032 PC patients, 55,683 (31%) underwent CT. Mean
age (SD) was 31.0 (23.6) years; 53% were female. In 2000, 178.5 CT
scans per 1,000 impaneled PC patients were performed, increasing to
195.9 in 2010 (10% absolute increase, p = 0.01). Although utilization
rates across the 10-year period remained stable, Emergency Department
(ED) CT examinations rose from 41.1 per 1000 in 2000 to 74.4 per 1000
in 2010 (81% absolute increase, p < 0.01). CT abdomen accounted for
over 50% of all CTs performed, followed by CT other (19%; included
scans of the spine, extremities, neck and sinuses), CT chest (16%), and
CT head (14%). Top diagnostic CT categories among those undergoing
CT were abdominal pain, lower respiratory disease, and headache.
Conclusions: Although utilization rates across the 10-year period
remained stable, CT use in the ED substantially increased. CT abdomen
and CT chest were the two most common studies performed and are
potential targets for interventions to improve the appropriateness of CT
use.
Key Words: computed tomography, x-ray, utilization, primary care
From 1Department of Emergency Medicine, Division of Emergency Medicine
Research, Mayo Clinic, Rochester, MN, USA; 2Knowledge and Evaluation
Research Unit, Mayo Clinic, Rochester, MN, USA; 3Department of Health
Sciences, Division of Health Care Policy and Research, Mayo Clinic, Rochester,
MN, USA; 4Department of Internal Medicine, Division of Primary Care Internal
Medicine, Mayo Clinic, Rochester, MN, USA; 5Texas Medical Institute of
Technology, Austin, TX, USA; 6Department of Radiology, Mayo Clinic,
Rochester, MN, USA
Correspondence: Erik P Hess MD, MSc, Mayo Clinic Rochester, 200 First St.
SW, Rochester, MN 55905, Phone: 507-284-8550, Fax: 507-255-6592 (fax)
Conflict of Interest Declaration: Dr. Charles Denham has served as an advisor to
GE Healthcare Services and Siemens. None of the other authors have any conflicts
of interest to declare. Funding support for this work was provided by a
philanthropic grant from Charles and Betsy Denham.
Copyright © 2012 by Lippincott Williams & Wilkins. Unauthorized reproduction
of this article is prohibited.
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The use of computed tomography (CT) has
increased dramatically over the past two decades,1 leading
to increased radiation exposure at the population level. In
2007 more than 69 million CT exams were performed in
the United States, and that number has increased nearly
10% annually.2 It has been reported that 80 million CT
scans are currently performed each year in the US.3,4 It is
estimated that CT examinations contribute up to 70% of
the total radiation dose to the population.5 This has led to
increasing public health concern regarding the potential
cancer risks associated with CT use.
Current risk estimates are derived largely from
data on atomic bomb survivors who were exposed to low
radiation doses that led to statistically significant
increases in cancer risk.6 An estimated 1.5 to 2 percent of
all cancer diagnoses in the US may be attributable to CT
use.7 Epidemiological evidence has demonstrated an
association between low-dose radiation exposure and the
development of solid tumors.8-11 Other data suggest
comparable but lesser estimates from excessive
radiation.12 Although the risks for any one person are not
large, the risk increases as additional studies are
performed, highlighting the need to keep radiation doses
as low as reasonably feasible when obtaining needed
diagnostic information and to limit unnecessary CT use.
Last year, the Joint Commission issued a
Sentinel Event Alert warning of the radiation risks of
diagnostic imaging.13 As pointed out in a recent article
entitled The No Outcome No Income Tsunami is Here, the
risk manager of the future must focus on anticipatory risk
identiﬁcation and mitigation systems. They must work
closely with the quality teams to develop early
recognition and warning initiatives that provide
measurable high-reliability execution.14 The modern risk
manager must utilize appropriate initiatives like National
Quality Forum (NQF) Safe Practice 4 (Risk Identiﬁcation
and Mitigation), which deﬁnes an entirely new
collaborative role between risk and performance
improvement groups.15
In the last few years, several studies have shown
a substantial increase in CT imaging over time.16,17 Most
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Hess et al ● Trends in CT Utilization Rates
of these studies have been conducted using crosssectional survey data or longitudinal payer data which do
not account for in-hospital imaging or the practice settings
in which these patients are managed. We are aware of no
published reports regarding trends in CT utilization in a
primary care population in an integrated, multispecialty
healthcare system or reports that highlight trends in CT
use by physician specialty in both the inpatient and
outpatient settings. The overall goal of this study was to
track trends in CT use in order to identify targets for
interventions to increase the appropriateness of CT use
across our multispecialty health system. Specifically, we
sought to: 1) investigate the trends in CT utilization in a
primary care population over a 10-year period (20002010); 2) describe patterns of CT ordering by physician
specialty; and 3) identify the leading types of CTs
obtained and the associated diagnoses.
METHODS
Study Design and Setting
We conducted a retrospective study of all
primary care patients from Mayo Clinic in Rochester,
MN, a large integrated, multispecialty group practice. We
focused on patients impaneled to primary care (Family
Medicine, Community Internal Medicine, and Pediatrics;
primary care impaneled patients are defined as those
patients assigned to a specific primary care provider).
This primary care practice serves local residents (from
Olmsted and surrounding counties), Mayo Clinic
employees, and their dependents.
Patient Selection
All pediatric and adult patients assigned to a
primary care provider from January 1, 2000 to December
31, 2010 were included in the analysis. Patients who did
not give consent for the use of their medical record were
excluded from analysis in accordance with Minnesota
state law. We assessed patterns of imaging in each year
from 2000 to 2010.
Data Collection and Processing
Information was electronically abstracted from
the electronic medical record (EMR) and administrative
databases within Mayo Clinic’s health records system.
Mayo Clinic maintains all electronic medical record
information within one system, including inpatient and
outpatient clinic-visit information. No manual chart
abstraction was performed.
Demographic variables collected included: date
of birth, gender, and insurance status. Age was grouped
into six categories of less than 18, 18 to 34, 35 to 49, 50
to 64, 65 to 79 and greater than 79. Insurance status was
grouped into five categories Medicare, no insurance, other
government
(Medicaid
and
state
programs),
private/contract and unknown.
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CT imaging procedures were extracted using
standardized Current Procedural Terminology, Fourth
Edition (CPT-4) codes. CT scans were organized by body
region into four categories: “head,” “chest,”
“abdominal,” and “other.” Abdominal CT included any
CT of the abdomen as well as any CT of the abdomen and
pelvis. Other CT included a variety of relatively
uncommon scans examining the spine, extremities, neck
and sinuses. Only CT scans performed on patients
paneled to a primary care provider were included. In
order to decrease the risk of overestimating utilization, we
collapsed multiple procedures done on the same region on
the same day into one CT. CT images performed on
patients while they were not paneled were not included.
All physicians who ordered a CT examination
for inpatient, outpatient, and emergency services from
2000 to 2010 were identified from the EMR. Physician
specialties were categorized as cardiology, emergency
medicine, gastroenterology, hematology, oncology,
primary care, urology, and other. The principal diagnosis
for each CT scan was obtained using diagnosis codes
from administrative billing data. We used the Agency for
Healthcare Research and Quality’s Clinical Classification
Software to organize the principal diagnoses into
diagnostic categories.18 Approval was obtained from the
Mayo Clinic Institutional Review Board prior to
conducting the study.
Statistical Analysis
Utilization rates per 1,000 impaneled patients
were calculated for baseline characteristics. Overall
utilization trends were examined by ordering physician
patterns and anatomical scanning region and the
frequency of common primary diagnosis by scanning
region over the entire study period.
In addition to standard descriptive statistical
methods, we performed linear regression analysis to
assess for trends by study month in CT imaging (number
of CTs per 1000 paneled patients). Analyses were
conducted with the use of SAS software, version 9.1
(SAS Institute, Cary, NC). Statistical significance was set
at .05 for modeling.
RESULTS
Our study population consisted of 179,032
patients who were impaneled to primary care between
2000 and 2010 (Table 1). Mean (±SD) age was
31.0 (±23.6) years, and 52.9% were female. A total of
228,121 CT scans were performed on 55,683 unique
primary care impaneled patients (31.1 %) during the 10year study period. Among those who had a CT scan
obtained, there was a mean of 4.1±5.4 scans and a median
of 2.0 scans per person (interquartile range, 1 to 5) over
the 10-year study period. The proportion of paneled
patients who underwent at least 1 CT increased with age,
ranging from 13.6% in patients less than 18 years of age
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Hess et al ● Trends in CT Utilization Rates
to 65.5% in patients greater than 79 years. A slight higher
percentage of females (32.3%) underwent CT scans
compared to men (29.7%). The portion of paneled
patients undergoing at least 1 CT was highest among the
Medicare group.
Table 1: Number of Impaneled Patients undergoing
a CT Scan, 2000 - 2010
All
Total Population
(N=179,032)
Patient having
had at least 1
CT
(N=55,683)
%
179,032
55,683
31%
Age Group
<18
61588
8421
14%
18-34
41815
10530
25%
35-49
35332
13395
38%
50-64
21845
11121
51%
65-79
13146
8740
66%
5306
3476
66%
31.0 (23.6)
44.3 (22.8)
>79
Mean Age
(SD)
-
Patient
Gender
Female
94759
30638
32%
Male
84273
25045
30%
Insurance
Status
Medicare
67%
No Insurance
5548
1462
26%
Other
Government
18999
5048
130239
36266
28%
869
50.0
40.0
27%
6266
Cardiologist
Emergency Medicine
Gastroenterologist
Hematologist
Oncologist
Other
Primary Care
Urologist
60.0
12038
Unknown
80.0
70.0
17980
Private/
Contract
Figure 1 displays the annual utilization rates of
CT imaging among our paneled primary care population
from 2000 to 2010. The annual utilization rate increased
from 178.5 CT scans per 1,000 in 2000 to 195.9 in 2010,
a 10% absolute increase (p = 0.01 for trend). Changes in
CT imaging rates per 1,000 impaneled patients are shown
in Table 2. The highest rate of increase in CT scans was
in the >79 year-old age group, which increased by 41%,
from 464.7 scans in 2000 to 645.5 in 2010 (p < 0.01 for
trend). The utilization rates for Medicare were much
higher than other insurance types and increased by 40%
from 2000 to 2010 (439.9 to 615.8, respectively). Overall,
the CT imaging rates were higher among females.
Table 3 reports CT utilization rates across study
years by body region. Overall the proportion of scans
was highest for CT Abdomen, accounting for nearly 50%
of all scans. CT abdomen had the largest increase relative
to other scan regions, rising from 83.8 per 1,000 paneled
patients in 2000 to 104.8 in 2010 (p < 0.01 for trend).
The most frequently occurring diagnoses for CT
by body region are listed in Table 4. Lower respiratory
disease and chest pain accounted for 50% of the CT scans
of the chest. Headache was the most frequent indication
for CT head (26%). More than 30% of CT abdomen
scans were for abdominal pain.
14%
30.0
20.0
10.0
10
08
09
20
20
06
07
20
20
05
20
04
20
03
20
01
02
20
20
250
20
20
00
0.0
200
Figure 2. Utilization rates per 1,000 Paneled Primary Care patients for all CT
imaging by ordering physician specialty from 2000 through 2010.
150
Figure 2 illustrates the overall utilization trends
by ordering physician specialty.
There was a notable trend in the growth of CT imaging
ordered by emergency medicine physicians. ED CT
examinations rose from 41.1 in 2000 to 74.4 in 2010
(81% absolute increase, p < 0.01), while the ED patient
volume remained stable. During the study period the
mean age of patients undergoing CT in both our
impaneled PC population and in those in the ED subgroup
remained stable (Appendix 1). By comparison, the CT
rate among primary care physicians decreased from 52.1
in 2000 to 38.3 in 2010 (p < 0.01 for trend).
100
50
0
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
Figure 1. Utilization rates of all CT imaging per 1,000 paneled
primary care patient, 2000 through 2010. The overall utilization rate increased
from 178.5 CT scans per 1,000 in 2000 to 195.9 in 2010, a 10% absolute increase
(p = 0.01 for trend).
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Table 5 reports the number of patients per year
who had received one, two, three and four or more CT
scans. Those receiving 4 or more scans in one year
increased from 9.6 per 1,000 paneled patients in 2000 to
13.2 in 2010 (p < 0.01 for trend).
DISCUSSION
Main study findings
The overall goal of this study was to track trends
in CT use across an integrated multispecialty health
system in order to identify targets for interventions to
increase the appropriateness of CT use. Though the
utilization rates across the 10-year period remained
relatively stable, CT use in the ED increased by over 80%
and was accompanied by a decrease in CT use by primary
care physicians. CT abdomen was the most frequently
evaluated region of the body and accounted for over 50%
of all CT scans performed, followed by CT other (scans
of the spine, extremities, neck, and sinuses), CT chest,
and CT head. The highest rate of CT scanning was
observed in patients over 79 years of age, and the top
indications for CT were abdominal pain, lung disease, and
headache.
The increasing use of CT scans involves many
medical specialties, of which the ED is an important
contributor. Over the study period we observed a marked
increase in the ordering of CT scans by emergency
medicine physicians. During this period, ED visits
remained stable, whereas the proportion of CT scans
ordered increased by 81 percent. Although one might
postulate that, as the population ages, patients cared for in
the PC and ED settings are increasing in age, potentially
explaining the increasing rate of CT utilization. However,
the mean age of patients undergoing CT in both the PC
and ED settings remained stable during the study period.
Given the large number of patients in the U.S.
being treated annually in EDs, this finding has enormous
implications for the general population.19 Our results
build on prior studies that suggest CT use in the ED may
be increasing sharply relative to its use in other clinical
settings.20-24 Some factors that may have contributed to
this increase in CT scans ordered by emergency
physicians are that CT scans may be more sensitive for
detecting serious injuries, the speed and timeliness to
obtain critical diagnosistic information, increased
availability of CT scanners and concern about malpractice
lawsuits for a missed diagnosis.25
CT Abdomen and CT Head are two of the most
commonly performed studies in the ED. Our detailed
analyses of diagnosis by CT body region found that
abdominal pain and kidney stones were the most frequent
diagnosis in CT Abdomen scans. We found that the most
common diagnosis in CT Head was headache. The
frequent use of CT for the diagnosis of common
conditions such as kidney stones and headaches has been
questioned in the literature.26-28 This analysis suggests that
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the utilization of CT Abdomen and CT Head in the ED
are potential opportunities to investigate strategies for
quality improvement. Several strategies have been
introduced in efforts to guide the appropriate use of
CTs.29 Numerous decision rules have been developed for
specific patient conditions such as minor head trauma,
pulmonary embolism and cervical spine injuries in an
effort to avoid potential over-utilization of CT scans.29-32
Our finding that some of our patients had more
than 4 CT scans per year is illustrative of the potential for
significant radiation accumulation over time. Exposure to
even moderate degrees of medical radiation presents an
important yet potentially avoidable public health threat.7
This underscores the need to ensure providers and patients
are informed about the risks of radiation and making sure
exposure is as low as reasonably achievable without
sacrificing quality of care.33 In a 2004 survey of
radiologists
and
emergency-room
physicians,34
approximately 75% of the group significantly
underestimated the radiation dose from a CT scan and
only 9% of emergency-room physicians believed that CT
scans increased the risk of cancer. To date there have
been recent efforts focusing on raising awareness among
providers about the potential risk of cumulative radiation
dose.35
Comparison with other published studies
A previous study focused on increases in use of
medical imaging using payer data and focusing on overall
imaging use in a large integrated health system.17 In this
study, the use of CT imaging increased from 81 per 1,000
enrollees to 181 per 1,000 enrollees between 1997-2006.
Similarly, use of outpatient CT imaging among Medicare
beneficiaries increased from 138 per 1,000 beneficiaries
to 292 per 1,000 beneficiaries between 1998 and 2008,
respectively.36
Our analysis extends this work by
specifically examining CT use across inpatient and
outpatient settings and CT use by ordering physician
specialty. As such, our finding that CT use has been
increasing in ED is of considerable interest.
Future Directions
With greater knowledge of CT utilization in our
PC population, providers and organizations may be
motivated to create strategies for the appropriate use of
CT scans. At Mayo Clinic, radiologists are now tracking
radiation exposure in order to better identify those
patients who have had multiple CT scans. This is the first
step to effectively work together with the ordering
physician and patients to reduce radiation exposure and
ensure that the most appropriate procedure is ordered.
Deployment of validated clinical decision support to
assist physicians in choosing the most appropriate
imaging study for the patient, is a promising area of
further investigation and has potential to decrease
unnecessary radiation exposure. The ED practice is a
logical environment for implementation and study of
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Hess et al ● Trends in CT Utilization Rates
computerized clinical support tools in light of the
significant increase in CT utilization in the ED.
and to measure the effect of interventions with
demonstrated efficacy on the diagnostic yield of imaging.
Limitations
CONCLUSIONS
Our investigation has several potential limitations. Our
data represent utilization from one health system, thus the
extent to which our findings can be generalized to other
populations is unknown. It is also likely that our imaging
rates are biased upwards somewhat since the payer-based
data included individuals who did not seek health care or
present to the health care system, and the denominator
only included patients impaneled in primary care. Our
cohort only included patients impaneled to a primary care
provider and excluded patients who did not have an
assigned primary care provider. However, one might
anticipate that patients presenting to the ED without an
assigned primary care provider might have an even higher
rate of CT utilization than that observed in this
investigation, as these patients have no established
mechanism to ensure outpatient follow-up. Moreover,
our data represent CT utilization in both the inpatient and
outpatient setting in a multispecialty health system and
thus provide a unique contribution to the existing
literature. There is potential for overestimating the
number of CTs when CTs were obtained on multiple body
regions on the same day. To decrease the risk of
overestimating utilization, we collapsed multiple
procedures done on the same region on the same day and
counted it as one CT. Finally, our study did not address
the appropriateness of CT use. We plan to conduct future
investigations that assess the appropriateness of CT use
CT utilization in this PC population remained
relatively stable over the study period. However, the
number of CT scans ordered by ED physicians increased
substantially. Abdominal CT was the most common type
of CT performed, followed by CT of other areas such as
chest and head.
NQF Practice 1, entitled “Leadership Structures
and Systems,” suggests that leadership structures and
systems should be established in hospitals to ensure that
there is organization-wide awareness of patient safety
performance gaps, direct accountability of leaders for
those gaps, and adequate investment in performance
improvement abilities, and that actions are taken to ensure
safe care of every patient served.15
These data obtained from all PC inpatients and
outpatients in an integrated multispecialty health system
identify opportunities for further improving CT utilization
and Safe Practice 1 implies that investment could be made
in further improvements to reduce radiation exposure.
Consideration of alternative nonionizing studies and
implementing evidence-based decision support –
particularly in the ED setting for patients with abdominal
pain and headache – are promising approaches for future
interventions to decrease radiation exposure, in patients
being considered for CT.
.
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Hess et al ● Trends in CT Utilization Rates
Table 2: CT Utilization rates per 1,000 Paneled Primary Care Patients between 2000 and 2010
2000
178.5
2001
166.9
2002
164.9
2003
177.9
2004
183.3
2005
193.3
2006
197.7
2007
191.7
2008
197.3
2009
193.9
p2010 value
195.9 0.01
<18
18-34
35-49
50-64
65-79
>79
52.8
124.5
167.2
252.1
401.7
464.7
46.2
118.8
160.6
244.0
390.5
473.8
45.2
116.1
159.2
243.3
411.4
492.3
45.4
131.9
179.5
273.0
447.7
493.4
39.6
123.9
187.2
278.4
485.3
571.9
43.4
126.3
196.3
287.1
524.5
611.0
42.9
128.1
194.5
294.7
517.1
635.2
39.5
115.8
184.3
292.6
516.7
632.3
36.2
124.4
198.1
281.6
529.2
644.2
35.2
123.2
201.1
270.3
507.5
606.1
30.4
120.9
196.3
282.1
494.9
645.5
Female
Male
Insurance
Status
Medicare
No Insurance
Other
Government
Private/Contract
Unknown
176.4
181.1
171.0
161.8
165.0
164.8
186.1
168.6
191.8
173.6
196.9
189.2
207.0
187.2
198.6
183.9
203.0
190.7
198.5
188.7
202.7 <0.01
188.2 0.08
439.9
211.6
172.8
451.5
244.2
164.3
483.4
213.0
193.6
521.7
247.4
219.8
583.5
235.6
272.1
633.1
192.0
263.2
628.2
174.4
284.5
639.4
175.3
268.6
653.9
206.0
265.8
613.1
198.3
284.9
615.8 <0.01
216.9 0.31
270.6 <0.01
125.4
0.0
128.6
0.4
135.3
0.0
155.0
0.3
160.1
0.2
171.4
0.1
169.3
2.0
163.2
1.0
163.0
1.1
155.3
1.0
156.5 0.08
0.8 <0.01
ALL
Age group
<0.01
0.87
<0.01
0.01
<0.01
<0.01
Sex
Table 3: Number of CT Scans performed (per 1,000 paneled patients) By Scan Region
ALL
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
pvalue
178.5
166.9
164.9
177.9
183.3
193.3
197.7
191.7
197.3
193.9
195.9
0.01
79.1
79.28
85.73
89.6
93.87
105.28
101.38
107.03
104.12
104.8
Image/Scan Region
CT
Abdomen
83.82
CT Chest
28.39
27.22
27.88
30.29
29.34
30.41
33.491
32.248
31.537
32.205
30.66
<0.01
CT Head
23.22
20.9
18.94
17.1
19.1
28.34
28.811
28.164
29.68
28.791
28.74
<0.01
CT Other
43.02
39.63
38.84
44.82
45.29
40.66
30.139
29.867
29.022
28.776
31.71
<0.01
6
<0.01
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Table 4. Top 5 Diagnostic Categories of CT scans by body
region, 2000-2010
CT Abdomen/Pelvis
N
%
Abdominal pain
33268
31
Calculus of urinary tract
8571
8
Genitourinary symptoms and ill-defined
conditions
Non-Hodgkins lymphoma
4184
4
3894
4
Other and unspecified gastrointestinal
disorders
CT Head
3360
3
N
%
Other headache
6000
26
Acute cerebrovascular disease
2133
9
Conditions associated with dizziness or
vertigo
Other nervous system symptoms and
disorders
Other intracranial injury
1708
7
1440
6
1267
5
CT Chest
N
%
Other and unspecified lower respiratory
disease
Nonspecific chest pain
12038
38
3854
12
Non-Hodgkins lymphoma
1781
6
Pulmonary heart disease
863
3
Pleurisy; pleural effusion
751
2
CT Other
Spondylosis; intervertebral disc disorders;
other back problems
N
%
4729
12
Fractures
4681
12
Respiratory infections
4105
10
Other upper respiratory disease
2835
7
Other injuries and conditions due to external
causes
2107
5
Table 5: Number Patients (per 1,000 paneled patients) by number of scans, 2000-2010
7
Number
of CT
Scans
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
1
43.9
42.6
37.5
37.6
35.5
32.7
38.0
36.8
36.6
35.8
34.9
0.02
2
28.3
26.9
26.5
28.3
28.4
31.4
30.6
29.3
29.8
29.2
29.7
0.01
3
7.2
6.4
6.6
6.8
7.1
8.2
7.8
7.4
7.6
7.5
7.2
0.01
4+
9.6
9.0
9.5
10.9
11.8
12.3
12.5
12.2
13.2
12.8
13.2
<0.01
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____________________________________________
Appendix 1. Mean age of the primary care (PC) impaneled population,
of PC impaneled patients who underwent computed tomography (CT),
and in those who underwent CT in the Emergency Department (ED) by
year.
Mean age
YEAR
Pre-Published Ahead of Print: 5/8/2012
Hess_Trends in CT Utilization Rates Longitudinal Study_JPS(8)2012_PrePub-v6.doc
PC
population
PC CT
ED CT
population
population
2000
36
54
52
2001
35
54
51
2002
35
54
51
2003
35
52
49
2004
35
54
50
2005
35
54
49
2006
35
54
50
2007
35
55
51
2008
36
55
51
2009
36
55
51
2010
36
56
53
© 2012 Lippincott Williams & Wilkins